The present invention generally concerns latching devices (i.e., latches) and, more particularly, magnetic latches.
The most common element designed to provide ON/OFF switching action when activated magnetically is a reed switch. As shown in FIG. 1, a normally-open reed switch 28 generally consists of two beams 30 disposed in a hermetically sealed glass cover 33. The beams 30 are made of magnetically permeable (i.e., soft) metal placed in close proximity to each other with a small gap between the ends (or contacts) 32 of the beams 30. When magnetic field of proper configuration is applied to the device, the beams 30 polarize magnetically such that they attract and form a mechanical and electrical contact. When the field is removed, the beams return to the initial state such that there is no electrical contact between the beams.
In order to polarize the beams in magnetically opposite states (to cause attraction between the beams), the field around the beams should be highly non-uniform. This is usually achieved by placing a magnetically hard dipole magnet in the proximity of the switch. The hardness of the magnet is defined as its resistance to re-magnetization (high coercive force, Hc, and high remnant magnetization, Mr). The beams of the switch are, in turn, very soft magnetically, i.e. they have very low Hc and very low Mr. This condition insures consistent and linear mechanical action, and prevents self-latching.
Magnetic latching devices (or “magnetic relays”) commonly include a reed switch. Such latching devices also typically include secondary solenoids which provide a field sufficient to retain the beams of the reed switch in the closed position, but insufficient to close the beams without an external field. Because the solenoids, however, require non-zero electrical current (or power), in circumstances when no such current can be provided, or it proves to be an excessive drain on a power supply, such magnetic latches are not practical for many applications. Accordingly, there exists a need for a passive magnetic latch.